1. Field of the Invention
The invention relates to ketone-aldehyde resins, especially cyclohexanone-formaldehyde resins, with low water content and high thermal stability and yellowing resistance, and to a process for preparing them and their use.
2. Description of the Background
It is known that ketones or mixtures of ketones and aldehydes can be reacted in the presence of basic catalysts or acids to form resinous products. Thus mixtures of cyclohexanone and methylcyclohexanone can be used to prepare resins (Ullmann Vol. 12, p. 551). Cyclohexanone and formaldehyde react to form hard resins which are used in the coatings industry.
Processes for preparing products of this kind are described in, for example, U.S. Pat. No. 2,540,885, U.S. Pat. No. 2,540,886, DD 12433, DE 13 00 256, DE 28 31 613, DD 123481 and DE 12 56 898.
The resins obtained in accordance with the various processes possess softening points, as apparent from an overview given in DD 123481, of up to 120° C.
DD 12433 describes a process for preparing gritlike products by addition of small amounts of swelling agents and thickeners or surface-active substances as dispersing assistants. Some or all of these additives may remain in the products, and consequently may have adverse effects on properties such as the water resistance of coatings from coating materials manufactured with the products. The skilled worker is also aware that the proportion of water in gritlike products is relatively high, since there may be water inclusions in the grit particles.
According to DD 123481 and DE 12 56 898 it is possible to obtain resins having softening points of up to 160° C. The processes described, however, are costly and inconvenient.
According to DE 28 31 613 polycondensation products are obtained from aliphatic or cyclic ketones or mixtures of these ketones with aliphatic aldehydes in the presence of basic catalysts, using from 0.005 to 10 mol %, based on the total amount of ketone and aldehyde, of a phase transfer catalyst. If the process disclosed therein is used to prepare a resin having a composition which corresponds to the present invention, high softening points and hence high molecular weights are obtained. Since the resins to which the invention relates are used, inter alia, in the coatings industry in order to reduce the solvent fraction, such high molecular weights are especially disadvantageous. The skilled worker is also aware that too high a molecular weight is detrimental to the broad solubility properties of cyclohexanone-formaldehyde resins. The skilled worker is further aware that resins, in accordance with the process disclosed therein, possess low nonvolatile fractions, as a result of which the thermal stability may be adversely affected and the yields of the process described are low.
DE 12 56 898 and U.S. Pat. No. 2,540,885 describe processes for preparing condensation products in the presence of inert solvents. The process on which the present invention is based succeeds without such additions, thereby obviating expensive reprocessing of said solvent and, for a given reactor size, allowing the use of greater quantities of reactants, so that higher conversions are achieved. The skilled worker is aware that resins, in accordance with the process described in U.S. Pat. No. 2,540,886, are obtained in low yields.
DE 13 00 256 describes a process for purifying ketone-formaldehyde resins. If the conditions there are applied to the cyclohexanone and formaldehyde condensation products of the invention the resultant resins, particularly as a 10% strength solution in xylene, contain an insoluble precipitate.
JP 47018866 adds an anhydride to a resin to obtain a low-water-content resin which can find use in polyurethane applications. The products of the invention have a low water content, suitable for polyurethane applications, without the addition of further substances.
JP 46004998 uses phenols in order to enhance the light stability and heat resistance of cyclohexanone-formaldehyde resins. The resins prepared by the process of the invention are stable without such additives.
At the present time a variety of cyclohexanone-formaldehyde resins are available on the market. The products differ primarily in their softening point (from 75–120° C.) and in their OH number (from 80–260 mg KOH/g).
Common to all these products is a fairly high water content of between 0.4% and 2.5% by weight. The high water content of the standard commercial products prevents their use in high-grade coating materials such as 2-component polyurethane varnishes, for example.
A further disadvantage of the standard commercial products is that solutions of these resins in aromatic solvents such as, say, xylene tend toward clouding owing to the high water content. These products are unsuitable for applications which use, for example, long-oil alkyd resins.
Furthermore, the resins presently available on the market possess a relatively low temperature stability, and so can be used only to a limited extent, if at all, in baking systems.